Polysaccharide-based superabsorbent film

ABSTRACT

A superabsorbent polysaccharide can be obtained by crosslinking a polysaccharide or derivative thereof with at least 1% by weight of a flexible spacer having a chain length of at least 9 chain atoms and having terminal activated coupling groups. The flexible spacer may comprise a polyalkyleneglycol with a molecular weight from about 400 to 10,000. The coupling groups may be provided by divinyl sulphone units.

FIELD OF THE INVENTION

The present invention relates to flexible superabsorbent films based onpolysaccharides such as cellulose and derivatives thereof, and to aprocess for producing such films.

BACKGROUND

Superabsorbent materials for use in hygiene products, which arc based onpolysaccharides such as cellulose and starch, have recently becomewidely known in the art, for example in WO 98/27117. The absorbingcapacity of such materials can be increased by crosslinking thepolymers, e.g. by using epichlorohydrin, diglycidyl ethers, divinylsulphone or other commonly known crosslinkers capable of reacting withhydroxyl groups, or by using carboxylated polysaccharides andcrosslinkers capable of reacting with carboxyl groups, such as divalentmetals. However, there is a demand for thinner absorbent products, whichimplies that superabsorbent materials have to be found which havefurther increased absorbing capacity and have increased flexibility.

WO 97/19116 describes superabsorbent acrylic polymers which arecrosslinlked by polymerisation of acrylic acid in the presence of acombination of trimethylolpropane triacrylate or triallylamine,polyethyleneglycol mono(meth)acrylate monoallyl ether andpolyethyleneglycol mono(meth)acrylate monoalkyl ether.

WO 97/31971 discloses similar, foamed superabsorbent acrylic polymerswhich are crosslinked e.g. with trimethytolpropane triacrylate, to whichinternal or external plasticisers (e.g. glycerol or acrylic esters) mayadded to increase flexibility of the foam.

DESCRIPTION OF THE INVENTION

It has been found that thin superabsorbent polysaccharides with highabsorption capacity and sufficient flexibility can be obtained bycrosslinking the polysaccharides with flexible spacers such aspolyalkyleneglycols, having terminal activated groups. The products andthe process of producing them are defined in the appending claims.

The polysaccharides to be used according to the present invention are inparticular α-glucans like starch, amylose and amylopectin, β-glucanslike cellulose, galactomannans like guar gum (guaran) and locust beangum, glucomannans including e.g. xanthan gum, fructans, (arabino)xylansand galactans, as well as derivatives such as carboxymethyl, alkyl,hydroxyethyl and hydroxypropyl derivatives of such polysaccharides.Cellulose and cellulose derivatives are preferred for practical reasons.Combinations of such polysaccharides, or combinations with otherpolymers such as polyacrylates, polyvinyl alcohol etc. can also be used.The chain length of the polysaccharides is important, although there isno critical minimum for the molecular weight. In general,polysaccharides having a molecular weight of more than 25,000 arepreferred.

The polysaccharides to be used according to the present invention mayalso be carboxymethylated or carboxyethylated, especially in the case ofcellulose. Other carboxyalkylated polysaccharides include the halfesters obtained from cyclic anhydrides such as succinic and maleicanhydride, and addition products of maleic half esters to which sulphitehas been added. The degree of carboxyalkylation is preferably between 0and 1.5, in particular between 0.1 and 1.0 carboxyalkyl groups permonosaccharide unit. The carboxyl derivatives may be in their acid or insalt form. Combinations of carboxylated polysaccharides such as CMC(carboxymethyl cellulose) and hydroxyallylated polysacharides (e.g.hydroxyethyl cellulose, HEC) are especially useful, whether as mixturesof different derivatives (e.g. HEC and CMC, or HEC and carboxymethylstarch, or HEC and methyl cellulose) or as multiply derivatised singlecompounds (e.g. sodium carboxymethyl-hydroxyethyl cellulose, CMHEC)

The polyalkyleneglycols to be used as spacers may for example bepolyethyleneglycol (PEG), polypropyleneglycol (PPG) and the like. Otherhydrophilic or hydrophobic spacers may also be used, as long as they areflexible, i.e. contain no or only a few double bonds or cyclicstructures; examples are polyalkylene (as in decamethylenediisocyanate), polyhydroxyalkylene, polyalkylene succinate, polylactide,etc, with chain lengths from about 9 to about 750 chain atoms. The chainlength of the polyalkyleneglycols may vary from edgy 3 units (MW about150 Da) up to e.g. 250 (MW about 11,000). Molecular weights from about1000 to about 8000 are preferred. The relative amount ofpolyalkyleneglycol with respect to the polysaccharide may vary fromabout 1/200 to about 1/1, especially from about 1/50 to about 1/1.5(weight ratios), depending on the required thickness and the requiredflexibility of the product.

The terminal activated groups are preferably vinyl groups activated bycarbonyl or sulphonyl functions, for example acryloyl groups(—CO—CR═CHR), maleoyl groups (—CO—CH═CH—COOH) or vinylsulphonyl groups(—SO₂—CR═CHR), in which each R may be hydrogen (preferred), methyl orother alkyl. Such groups may be directly attached to thepolyalkyleneglycol, e.g. as (sulphonate) esters, or through alkylene orphenylene groups. Particularly advantageous is the coupling product of apolyalkyleneglycol with divinyl sulphone on either side of thepolyglycol. Other terminal crosslinkers include (activated) halomethyl,activated hydroxymethyl, activated formyl, epoxy, isocyanate, and thelike. Examples of such coupling agents (other than divinyl sulphone) aremaleic anhydride, dichloroacetone, 1,3-dichloro-2-propanol,dimethylolurea, dimethylolimidazolidone, diepoxides such asbisepoxybutane or bis(glycidyl ether), epichlorohydrin, diisocyanates,bis(2-hydroxyethyl) sulphone, formaldehyde, glyoxal. The weight ratiobetween terminal crosslinker (such as divinyl sulphone) and spacer (suchas polyalkylene glycol) can be between about 1/1 to about 100/1,especially between about 1.5/1 and 30/1. The weight ratio betweencrosslinker and polysaccharide may vary from e.g. 1/1 to 1/50,especially form 1/1.5 to 1/10.

The production of the superabsorbent films according to the inventioncan be divided in three steps: (1) mixing of reactants and othercompounds, (2) reaction and washing stage, and (3) desiccation. As tostep (1), the components involved in the reaction can be divided indifferent classes: (a) components of the base structure of the network,i.e. the polysaccharides, e.g. carboxymethyl cellulose sodium salt(CMCNa) and/or hydroxyethyl cellulose (HEC); (b) crosslinkers, e.g.divinyl sulphone (DVS); (c) spacers, e.g. polyethylene glycol (PEG); (d)catalysts, e.g. KOH; and solvents, e.g. water. In step (2), thereactants are allowed to react for a sufficient time to allow theproduction of a crosslinked gel. Preferably, the polyalkylene glycol andthe reagent introducing the terminal double bonds are reacted first,followed by reaction with the polysaccharide, preferably in the presenceof an alkaline catalyst. The crosslinking reaction can be performed atvarying temperatures e.g. from about 5° C. to about 40° C., for about 1hour to about 2 days, preferably form 5–24 hours. After thecrosslinking, the unreacted reagents can be removed by washing indistilled water, if desired, followed by drying. The crosslinked productcan also be directly dried without a washing step.

The superabsorbent products according to the invention are flexiblefilms with thicknesses between 10 and 500 μm and having absorptioncapacities between about 15 and 30 g of synthetic urine (300 mM urea, 60mM KCl, 130 mM NaCl, 2.0 mM CaSO₄, 3.5 mM MgSO₄, 29 MM KH₂PO₄, 5.3 MMNa₂HPO₄, 1 mg/l Triton X-100 in deionised water) per g of product. Theycan be used in absorbent articles, such as diapers, incontinence guards,sanitary napkins, and the like. They can also be used in tissue papersincluding kitchen towels, napkins, industrial wipes and the like.

EXAMPLES

Materials: Divinyl sulphone (DVS), polyethyleneglycol (PEG) with variousmolecular weights (400, 4600, 10,000), hydroxyethyl cellulose (HEC, MW250,000) and carboxymethyl cellulose (CMCNa, MW 700,000) were obtainedfrom Aldrich Chimica, Milano, IT.

The amounts of reagents are given in the tables, per 150 ml of distilledwater. DVS was dissolved in distilled water to a concentration of 40mmol/l. PEG was then added to the DVS solution. After dissolution of thePEG the CM CNa and HEC were added in powder form and dissolved up to aconcentration of about 2% (see tables). Best results were obtained byfirst dissolving HEC and then slowly admixing CMCNa. Mixing wascontinued at 25° C. until a clear solution was obtained. After completemixing, 1M of aqueous KOH was dissolved into the mixture up to thedesired concentration. After another two minutes of stirring thereaction mixture was spread on a teflon sheet with a Gardner knife inorder to obtain a film with a controlled thickness. The film was allowedto crosslink at ambient temperature for between 5 and 24 hours (bestresults after 10–14 hours). Higher temperatures did not increase thecrosslinking rate, and resulted in decreased viscosity. A thin,partially swollen gel film was obtained.

From this point on, two different procedures were followed. According tothe first procedure, the teflon sheet with the partly swollen film wasthen put in a jar containing distilled water. As soon as the filmstarted to swell further, the teflon sheet was removed. During swelling,water mixture containing residual KOH, unreacted DVS and otherimpurities was continuously removed from the bottom of the jar, whilefresh distilled water was added. After equilibrium swelling occurred,the teflon sheet was again positioned under the film, water around thefilm was removed and the film was dried under atmospheric conditions.

According to the second procedure, the washing (addition and removal ofwater) was omitted and the swelling film was maintained on the teflonsheet for 5–24 hours and then dried under atmospheric conditions.

As an alternative to drying under atmospheric conditions (for about 6–20days), desiccation was performed in an oven at 50–100° C., with bestresults being obtained at 60–80° C., for 1–24 hours.

TABLE 1 Hydrogel synthesis mixture with PEG 400 Molar ratio [PEG]/[DVS]= 1/30; molar ratio [PEG]/[cellulose] = 16/1 Reagent grams mmoles % byweight Water 150 8330 + 280¹ 94.54 CMCNa 2.25 3.21 * 10⁻³ 1.42 HEC 0.753.00 * 10⁻³ 0.47 KOH, 1 M in water 5.28 KOH: 5.00 3.33 DVS 0.35 2.960.22 PEG 400 0.04 0.100 0.03 ¹the water of the KOH solution

TABLE 2 Hydrogel synthesis mixture with PEG 400 Molar ratio [PEG]/[DVS]= 1/90; molar ratio [PEG]/[cellulose] = 11/1 Reagent grams mmoles % byweight Water 150 8330 + 280¹ 94.33 CMCNa 2.25 3.21 * 10⁻³ 1.41 HEC 0.753.00 * 10⁻³ 0.47 KOH, 1 M in water 5.28 KOH: 5.00 3.32 DVS 0.71 6.010.45 PEG 400 0.027 0.0675 0.02 ¹the water of the KOH solution

TABLE 3 Hydrogel synthesis mixture with PEG 400 Molar ratio [PEG]/[DVS]= 1/60; molar ratio [PEG]/[cellulose] = 16/1 Reagent grams mmoles % byweight Water 150 8330 + 280¹ 94.32 CMCNa 2.25 3.21 * 10⁻³ 1.41 HEC 0.753.00 * 10⁻³ 0.47 KOH, 1 M in water 5.28 KOH: 5.00 3.32 DVS 0.71 6.010.45 PEG 400 0.04 0.100 0.03 ¹the water of the KOH solution

TABLE 4 Hydrogel synthesis mixture with PEG 400 Molar ratio [PEG]/[DVS]= 1/10; molar ratio [PEG]/[cellulose] = 96/1 Reagent grams mmoles % byweight Water 150 8330 + 280¹ 94.20 CMCNa 2.25 3.21 * 10⁻³ 1.41 HEC 0.753.00 * 10⁻³ 0.47 KOH, 1 M in water 5.28 KOH: 5.00 3.32 DVS 0.71 6.010.45 PEG 400 0.24 0.600 0.15 ¹the water of the KOH solution

TABLE 5 Hydrogel synthesis mixture with PEG 400 Molar ratio [PEG]/[DVS]= 1/200; molar ratio [PEG]/[cellulose] = 16/1 Reagent grams mmoles % byweight Water 150 8330 + 280¹ 93.36 CMCNa 2.25 3.21 * 10⁻³ 1.40 HEC 0.753.00 * 10⁻³ 0.47 KOH, 1 M in water 5.28 KOH: 5.00 3.29 DVS 2.35 19.91.46 PEG 400 0.04 0.100 0.03 ¹the water of the KOH solution

TABLE 6 Hydrogel synthesis mixture with PEG 400 Molar ratio [PEG]/[DVS]= 1/100; molar ratio [PEG]/[cellulose] = 32/1 Reagent grams mmoles % byweight Water 150 8330 + 280¹ 93.34 CMCNa 2.25 3.21 * 10⁻³ 1.40 HEC 0.753.00 * 10⁻³ 0.47 KOH, 1 M in water 5.28 KOH: 5.00 3.29 DVS 2.35 19.91.46 PEG 400 0.08 0.200 0.05 ¹the water of the KOH solution

TABLE 7 Hydrogel synthesis mixture with PEG 4600 Molar ratio [PEG]/[DVS]= 1/30; molar ratio [PEG]/[cellulose] = 16/1 Reagent grams mmoles % byweight Water 150 8330 + 280¹ 94.29 CMCNa 2.25 3.21 * 10⁻³ 1.41 HEC 0.753.00 * 10⁻³ 0.47 KOH, 1 M in water 5.28 KOH: 5.00 3.32 DVS 0.35 2.960.22 PEG 400 0.46 0.100 0.29 ¹the water of the KOH solution

TABLE 8 Hydrogel synthesis mixture with PEG 4600 Molar ratio [PEG]/[DVS]= 1/60; molar ratio [PEG]/[cellulose] = 16/1 Reagent grams mmoles % byweight Water 150 8330 + 280¹ 94.07 CMCNa 2.25 3.21 * 10⁻³ 1.41 HEC 0.753.00 * 10⁻³ 0.47 KOH, 1 M in water 5.28 KOH: 5.00 3.31 DVS 0.71 6.010.45 PEG 400 0.46 0.100 0.29 ¹the water of the KOH solution

TABLE 9 Hydrogel synthesis mixture with PEG 4600 Molar ratio [PEG]/[DVS]= 1/33; molar ratio [PEG]/[cellulose] = 96/1 Reagent grams mmoles % byweight Water 150 8330 + 280¹ 91.80 CMCNa 2.25 3.21 * 10⁻³ 1.38 HEC 0.753.00 * 10⁻³ 0.46 KOH, 1 M in water 5.28 KOH: 5.00 3.23 DVS 2.35 19.91.44 PEG 400 2.76 0.600 1.69 ¹the water of the KOH solution

TABLE 10 Hydrogel synthesis mixture with PEG 10,000 Molar ratio[PEG]/[DVS] = 1/30; molar ratio [PEG]/[cellulose] = 16/1 Reagent gramsmmoles % by weight Water 150 8330 + 280¹ 93.97 CMCNa 2.25 3.21 * 10⁻³1.41 HEC 0.75 3.00 * 10⁻³ 0.47 KOH, 1 M in water 5.28 KOH: 5.00 3.31 DVS0.35 2.96 0.22 PEG 400 1.00 0.100 0.63 ¹the water of the KOH solution

1. A process for producing a crosslinked flexible superabsorbentpolysaccharide, comprising reacting a polyalkyleneglycol with at leasttwo equivalents of a reagent containing one or more activated doublebonds selected from the group consisting of divinyl sulphone, maleicanhydride, dichloroacetone, 1,3-dichloro-2-propanol, dimethylolurea,dimethylolimidazolidone, diexpoides, epichlorohydrin, diisocyanates,bis(2-hydroxyethyl) sulphone, formaldehyde, and glyoxal, to form acrosslinking spacer having a chain length of at least 9 chain atoms andhaving terminal activated coupling groups and reacting at least 1% byweight of the crosslinking spacer with a polysaceharide or a derivativethereof selected from the group consisting of carboxymethyl, alkyl,hydroxyethyl and hydroxypropyl derivatives in the presence of acatalyst.
 2. A hygiene product containing a crosslinked flexiblesuperabsorbent polysaccharide film produced according to the process ofclaim
 1. 3. The process according to claim 1, wherein said reagentcomprises divinyl sulphone.
 4. A crosslinked flexible superabsorbentpolysaccharide produced by the process claim
 1. 5. The crosslinkedflexible superabsorbent polysacoharide according to claim 4, whereinsaid polysaccharide has a molecular weight, before crosslinking, ofbetween 250,000 and 1,000,000 kD.
 6. The crosslinked flexiblesuperabsorbent polysaccharide according to claim 4, in which saidpolyalkyleneglycol has a molecular weight from about 400 to 10,000 kD.7. The crosslinked flexible superabsorbent polysaccharide according toclaim 4, in which said polyalkyleneglycol is polyethyleneglycol.
 8. Thecrosslinked flexible superabsorbent polysaccharide according to claim 4,in which said coupling groups comprise vinyl sulphone groups.
 9. Thecrosslinked flexible superabsorbent polysaccharide according to claim 4,in which 10–67% by weight of the crosslinking spacer, with respect tothe polysaccharide, has been used.
 10. The crosslinked flexiblesuperabsorbent polysaccharide according to claim 4, in which saidpolysaccharide has a molecular weight, before crosslinking, of between100,000 and 1,500,000 kD.
 11. The crosslinked flexible superabsorbentpolysaccharide according to claim 4, which has the form of a film havinga thickness of between 10 and 500 μm.
 12. A hygiene product containing acrosslinked flexible superabsorbent polysaceharide film according toclaim
 11. 13. A process for producing a crosslinked flexiblesuperabsorbent polysaccharide comprising reacting a spacer selected fromthe group consisting of polyalkyleneglycol, polyalkylene, decamethylenediisocyanate, polyhydroxyalkylene, polyalkylene succinate, andpolylactide with at least two equivalents of a reagent containing one ormore activated double bonds to form a crosslinking spacer having a chainlength of at least 9 chain atoms and having terminal activated couplinggroups selected from the group consisting of acrylol groups, maleoylgroups, and vinylsulphonyl groups and reacting at least 1% by weight ofthe crosslinking spacer with a polysaccharide or a derivative thereofselected from the group consisting of carboxymethyl, alkyl, hydroxyethyland hydroxypropyl derivatives in the presence of a catalyst.
 14. Acrosslinked flexible superabsorbent polysaccharide produced by theprocess of claim
 13. 15. The crosslinked flexible superabsorbentpolysaccharide according to claim 14, in which said activated couplinggroups are selected from the group consisting of acrylol groups, maleoylgroups, and vinylsulphonyl groups.
 16. The crosslinked flexiblesuperabsorbent polysaccharide according to claim 14, in which saidcoupling groups comprise vinyl sulphone groups.
 17. The crosslinkedflexible superabsorbent polysaccharide according to claim 14, in which10–67% by weight of the crosslinking spacer, with respect to thepolysaccharide, has been used.
 18. The crosslinked flexiblesuperabsorbent polysaccharide according to claim 14, in which saidpolysaccharide has a molecular weight, before crosslinking, of between100,000 and 1,500,000 kD.
 19. The crosslinked flexible superabsorbentpolysaccharide according to claim 14, which has the form of a filmhaving a thickness of between 10 and 500 μm.
 20. A hygiene productcontaining a crosslinked flexible superabsorbent polysacoharide filmaccording to claim 19.